Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox.
An important auxiliary system generally provided on wind turbines is a pitch system. Pitch systems are employed for adapting the position of a wind turbine blade to varying wind conditions by rotating the blade along its longitudinal axis. In this respect, it is known to rotate a wind turbine blade in such a way that it generates less lift (and drag) when the wind speed increases. This way, even though the wind speed increases, the torque transmitted by the rotor to the generator remains substantially the same. It is furthermore also known to rotate wind turbine blades towards their stall position (so as to reduce the lift on the blades) when the wind speed increases. These wind turbines are sometimes referred to as “active-stall” wind turbines. Pitching may furthermore also be used for rotation of the blade towards its vane position, when a turbine is temporarily stopped or taken out of operation for e.g. maintenance.
A common control strategy of a variable speed wind turbine is to maintain the blade in a predefined “below rated pitch position” at wind speeds equal to or below nominal wind speed (for example from approximately 3 or 4 m/s to 15 m/s). Said default pitch position may generally be close to a 0° pitch angle. The exact pitch angle in “below rated” conditions depends however on the complete design of the wind turbine. In the lower wind speed regions (at “partial load”), the objective is generally to maximize power output by maintaining pitch constant, thereby catching maximum energy, and varying generator torque and the rotor speed to keep the power coefficient, Cp, at a maximum. Above the nominal speed (for example from approximately 10 m/s to 25 m/s), the blades are rotated to maintain the aerodynamic torque delivered by the rotor substantially constant. Cut-in wind speed may e.g be around 3 m/s, nominal wind speed may be e.g. around 10 m/s and cut-out wind speed may e.g. be around 25 m/s. The nominal wind speed, cut-in wind speed and cut-out wind speed may of course vary depending on the wind turbine design. Said wind speeds may be measured typically at hub height.
Often, a doubly fed induction generator (DFIG) is used on variable speed wind turbines. In these DFIG's, the generator rotor is connected to the grid through a power electronics converter. Such a converter may comprise a Grid-Side-Converter (GSC), a DC link, and a Machine-Side-Converter (MSC). An advantage of using a DFIG with converter in wind turbines is that reactive power can be exported and imported from the converter. Also, through the control over rotor currents and voltages, synchronization with the grid is possible even though the rotor speed varies. Furthermore, a DFIG allows reducing the capacity and dimensions of the converters used.
It is a general goal to try to maximize electricity generation from cut-in wind speed to cut-out wind speed. However, in certain wind turbines with particularly large blades, when the wind speeds reaches a predetermined value above nominal wind speed, the loads on the blades may need to be reduced in order to ensure structural integrity of the wind turbine. Alternatively, the whole of the wind turbine needs to be structurally reinforced to such a point that the Cost-of-Energy (COE) would increase.
One reason for employing relatively large blades may be the limited number of suitable sites for wind turbines. It has become practice in the last couple of years to design and manufacture new wind turbine models which are based on older models, but in which new longer blades are used. In these cases, it may be that only the blades are substituted whereas the other components or many other components of the wind turbine stay the same. This may be useful particularly for generating more electricity at predominant wind speeds. However, at relatively high wind speeds, this kind of configuration could cause structural problems due to high loads. Therefore, the operational range of the wind turbine may be adjusted, i.e. reducing the cut-out wind speed, or the rotor speed of the wind turbine may be reduced at high wind speeds.
In practice, wind turbine designers and manufacturers need to balance the COE, the operational range of the wind turbines and the power that can be generated. The present invention aims at providing a method of operating a wind turbine that improves the balance, e.g. a method that allows increasing the operation range and/or the power generated without substantially increasing the COE.